1. Field of the Invention
The present invention relates to an optical power splitter and polarization splitter and, more particularly, to a 3.times.3 light guiding arrangement wherein light containing both TE and TM polarization may be launched in the middle light guide to form, for example, outputs of TM/2 in each of the two outer guides and TE in the center guide.
2. Description of the Prior Art
There exist may systems which require both optical beam splitters and polarizers. For example, effective beamsplitters are often a useful component in laser sharing systems, for example, optical customer loop networks must share laser sources to meet the needs of numerous customers with a minimal cost. In these instances, 1.times.N beamsplitters could be utilized to share a single laser among a plurality of N users. Another candidate for optical beamsplitters are rotation measuring fiber optic gyroscopes, which are beginning to replace mechanical gyroscopes for both military and civilian applications. In a fiber gyro, a light beam is launched in counter propagating directions through a coil of fiber, preferably single mode fiber. As the system rotates, to the light beam traveling in the same direction with the rotation will become delayed with respect to the light beam traveling against the direction of rotation. That is, the apparent optical path length seen by the light traveling with the rotation is increased, while the apparent optical path length seen by the light traveling against the rotation is decreased. The increased and decreased apparent optical lengths thus result in a phase difference between the two light beams as they emerge from the fiber loop. This phase difference can then be measured to to provide an indication of the system's rotation.
A typical prior art optical gyroscope is disclosed in U.S. Pat. No. 4,280,766 issued to W. C. Goss et al on July 28, 1981. In the Goss et al system, a 2.times.2 beamsplitter is used to create the pair of light beams which will propagate in opposite directions through the fiber loop. A problem with the 2.times.2 beamsplitter arises in systems which require a high degree of sensitivity. It can be shown that in order to provide a linear relationship between rotation and phase difference, an additional phase bias of .pi./2 must be introduced into the system. A complete explanation of this aspect of 2.times.2 beamsplitters can be found in the article entitled "Fiber-optic gyroscope with [3.times.3] directional coupler" by S. K. Sheem appearing in Applied Physics Letters, Vol. 37, No. 10, November 1980 at p. 869-871. Many 2.times.2 optical gyroscope systems, including Goss et al, utilize external components to provide this phase bias. In his article, Sheem proposes an alternative solution, the use of a 3.times.3 beamsplitter, which can be demonstrated to incorporate this required phase bias. U.S. Pat. No. 4,440,498 issued to S. K. Sheem on Apr. 3, 1984 discloses an optical gyroscope utilizing the 3.times.3 directional coupler described in the article. In this system, the incoming light is launched into the center waveguide and the outer pair of waveguides are coupled to the fiber loop forming the gyro. As with the system discussed above, the phase difference between the light outputs from the outer pair of waveguides is indicative of the rotation experienced by the gyroscope. Various alternative configurations of the 3.times.3 directional coupler are discussed by Sheem in a later article entitled "Optical fiber interferometers with [3.times.3] direction couplers: Analysis" appearing in the Journal of Applied Physics, Vol. 52, No. 6, June 1981 at pp. 3865-3872. One such alternative arrangement is disclosed in U.S. Pat. No. 4,479,715 issued to Sheem on Oct. 30, 1984, wherein a combination of a 3.times.3 directional coupler with a 2.times.2 directional coupler are utilized to form a rotation-sensing interferometer. Another implementation, which can be formed as a planar structure, is described in an article entitled "3.times.2 Channel Waveguide Gyroscope Couplers: Theory" by W. K. Burns et al appearing in the IEEE Journal of Quantum Electronics, Vol. QE-18, No. 10, October 1982 at pp. 1790-96. In this type of coupler, the incoming central waveguide is terminated in the body of the structure, thus decreasing the separation between adjacent optical paths to increase the amount of optical coupling. U.S. Pat. No. 4,445,780 issued to W. K. Burns on May 1, 1984 discloses a rotation-sensing gyroscope utilizing this 3.times.2 coupler.
Since gyroscopes require a unique path for the two counterpropagating beams, polarizing components must also be utilized. An imperfect polarizer in the gyroscope results in a phase bias which is proportional to the extinction ratio of the polarizer. False readings of rotation rate result due to this phase bias. Often, these polarizers are formed by incorporating predetermined bends in the fiber loop forming the rotator. These bends place a stress on the fiber which induces a birefringence in the fiber, thus altering the propagating constants of the two polarized modes and insuring a predetermined degree of coupling between the two modes. However, the amount of bending required to provide the necessary stress-induced birefringence will change as a function of type of fiber utilized, the size of the fiber loop, the number of turns used to form the loop, etc. Additionally, system changes may disturb the fiber loop so as to change the induced birefringence. For example, military system applications may introduce changes into the fiber loop which alter or destroy this induced birefringence. Alternative polarizing arrangements include loading the waveguide with either an evaporated material or an optically contacted (and birefringent) superstrate. These arrangements, although useful, are found in practice to exhibit a poor degree of polarization extinction as a function of length, and offer no design freedom in the choice of polarization which is suppressed.
Therefore, a need remains in the prior art for an integrated component capable of performing both the power splitting and polarization splitting required for fiber gyroscope applications.